I managed to find it. Took me some time and various experimentations. But it ended up being kind of straightforward.
For anyone else reading who wants a solution.
What I did was play level 02 Restore the balance and let enemies fall into the puddle until I got to -100 (going to -16 might me enough too)
The reason why this solution eluded me for so long is because this is what I did to complete level 2 in the first place. I don't even know if there's another solution.
I managed to fully complete the game now, it's pretty fun searching for secrets. I was hoping for an extra secret in the ending screen (like in the hard boiled levels) but there wasn't one (that I was able to find)